Bright stock oil is a Group I base oil which has undergone solvent extraction, dewaxing and optionally a hydrogen treatment. It is commonly used as a base oil in lubricating oil compositions, in particular in lubricating oil compositions for marine and stationary low-speed crosshead diesel engines burning residual fuels with sulphur contents of up to 3.5 wt % and for trunk piston, medium-speed diesel engines operating on residual fuel in industrial and marine applications. However, as bright stock oil availability is becoming increasingly constrained within the market place, it is desirable to find alternative base oils for use in lubricating oil compositions for such applications.
EP 1752514 discloses that particular base oil blends comprising bright stock oil and de-asphalted cylinder oil (DACO) not only have no adverse effect on the lubricating properties of lubricating oil compositions, but also have advantageous viscometric properties in cylinder oil lubricants for use in cross-head engines and trunk piston engines. These lubricating compositions still comprise bright stock oil and it is desirable to further reduce the proportion of bright stock oil in lubricating compositions.
The present inventors have sought to provide lubricating composition suitable for use in two-stroke crosshead engines that contain even less bright stock oil, and preferably no bright stock oil. Preferably such lubricating compositions have advantageous properties such as reduced deposit formation.